1. Field of the Invention
The present invention balanced signal receiving circuit provided with a differential amplifier which receives a transmitted balanced signal as an input. The signal receiving circuit has a step type input-output characteristic such that the logic of the output inverts when the input voltage crosses the zero volt level. An alarm generating means issues an alarm signal when the transmission of the balanced signal is stopped.
Discussion will be made of a balancing transmission system having a balanced signal transmitting circuit which transmits the balanced signal and a balanced signal receiving circuit which receives the balanced signal through a paired line.
In this system, when the balanced signal transmitting circuit and the balanced signal receiving circuit are installed far apart from each other, generally a special signal line for immediately notifying the receiving circuits of abnormalities occurring at the transmitting circuit side is not necessary. The "abnormalities" spoken of include stopping the transmission of the balanced signal, for example, due to a transmission side power down at the transmitting circuit side or disconnection of the paired line connector at the transmitting circuit side. Therefore, in the above-mentioned balanced signal receiving circuit, it is necessary to provide an alarm generating means for issuing an alarm signal when the transmission of the balanced signal stops.
2. Description of the Related Art
Normally, a terminating resistor is provided at the input of the above differential amplifier. This terminating resistor has a very low resistance value, so when the above-mentioned abnormality occurs, the input of the differential amplifier short-circuits. This results in the output level of the balanced signal receiving circuit becoming indefinite. Therefore, the alarm signal can no longer be generated.
To eliminate this problem, a pull-up resistor and a pull-down resistor are connected in series with the terminating resistor at the input of the differential amplifier. The pull-up resistor and pull-down resistor stabilize the output level of the balanced signal receiving circuit and enable reliable generation of the alarm signal by the stabilized output level.
If use is made of an alarm generating means including the pull-up resistor and the pull-down resistor, a deleterious electromagnetic wave is radiated from a transmission line (paired line) of the balanced signal, which works as a source of electromagnetic interference (EMI). Note that the above prior art and problems therewith will be explained in more detail later with reference to the drawings.